Recently, regarding an ultrasonic diagnostic device that performs image diagnosis with respect to an object by using ultrasonic waves, studies have been made about a configuration for compressing, and recording or transmitting data of reflected wave signals obtained from a probe. For example, reflected wave signal data obtained from a probe are compressed and thereafter transmitted to a device main body through a cable, while in the device main body, and conversely, the compressed data are decompressed so that the original reflected wave signal data are reproduced, whereby a predetermined image display is performed. By so doing, the reduction of transmitted data amount and the high-speed transmission of data are enabled.
There are ultrasonic-diagnosis-use probes of various shapes and various types, among which a convex-type probe having a sectoral range of ultrasonic scanning as shown in FIG. 17 is used widely. In FIG. 17, the range of scanning with use of ultrasonic waves emitted from a probe 101 toward the inside of a measurement object 102 has a sectoral shape that is widened as the depth increases. In the ultrasonic scanning range, straight lines of ultrasonic waves are emitted from the probe 101 toward the inside of the measurement object 102 with the angle of emission being varied gradually as shown in FIG. 17, and these lines are called acoustic lines. There are n acoustic lines 103 (103-1 to 103-n), from the left end to the right end of the sectoral scanning range.
FIG. 18 schematically illustrates the structure of reflected wave signal data of one frame. As shown in FIG. 18, acoustic line data 104-1 to 104-n corresponding to the number n of acoustic lines 103 are stored in a vertical direction in sequence. In each frame (acoustic line data group) 104, a datum closer to the right end is a datum at a greater depth.
FIG. 19 schematically illustrates an ultrasonic image displayed according to a sectoral scanning range. When the intensity of a reflected wave signal obtained from a probe is displayed on a screen 105, an ultrasonic image obtained by modulating an intensity of a reflected signal is displayed in a sectoral area 106 corresponding to a scanning range, as shown in FIG. 19. This display mode is called “B-mode”. This display mode allows intuitive diagnosis since an ultrasonic image in a sectoral range identical to a scanning range is displayed on a screen.
For such an ultrasonic image display, the display is performed after modifying image data of one frame as shown in FIG. 18 into a sectoral display range as shown in FIG. 19. Here, since the density of pixels on the screen is uniform, the acoustic line density in the displayed screen varies with the depth, as is understandable from FIG. 19. The acoustic line density is indicative of a value obtained by dividing the total number of acoustic lines by the number of display pixels. For example, assuming that the number of display pixels along an arc corresponding to a part of the smallest depth (the part closest to the center of the sector) is m1 and the acoustic line density of the foregoing part is A, A=n/m1 is yielded. Assuming that the number of display pixels along an arc corresponding to a part of the greatest depth (the part farthest to the center of the sector) is m2 and the acoustic line density of the foregoing part is B, B=n/m2 is yielded. m1<m2, and hence A>B. Thus, as the depth increases (the proximity to the center of the sector decreases), the acoustic line density decreases.
Further, as a method for compressing and encoding static image data, the JPEG method has been predominant conventionally, and it is possible to apply the JPEG method to the data compression of reflected wave signal in an ultrasonic diagnostic device.
FIG. 20 is a block diagram illustrating a schematic configuration of a conventional static image compression unit typically using the JPEG method. In FIG. 20, 11 denotes a block divider, 12 denotes a DCT (discrete cosine transformation) portion, 13 denotes a quantizer, 14 denotes an encoder, and 15 denotes a quantization factor output portion. Further, FIG. 21 is a block diagram illustrating the internal configuration of the quantization factor output portion 15 in the image compression unit shown in FIG. 20. In FIG. 21, 18 denotes a basic quantization table in which values that the quantization factors (values indicative of the fineness of quantization) are based on are shown in accordance with the sizes of blocks, and 19 denotes a multiplier that outputs a quantization factor derived by multiplication of a value obtained from the basic quantization table 18 by a preset scale factor (parameter for adjusting a compression ratio).
As shown in FIGS. 20 and 21, first, image data inputted are divided into 8×8-pixel blocks by the block divider 11. Each block is subjected to DCT by the DCT portion 12. The DCT coefficient outputted as a result of the DCT is quantized by the quantizer 13 according to the quantization factor given by the quantization factor output portion 15, and is converted into a Huffman code by the encoder 14, whereby compressed data are obtained.
FIG. 22 is a schematic block diagram showing a conventional static image decompression unit typically using the JPEG method. In FIG. 22, 31 denotes a decoder for decoding Huffman codes, 32 denotes an inverse quantizer for performing inverse quantization, 33 denotes an inverse DCT portion for applying inverse DCT, and 34 denotes an inverse quantization factor output portion.
As shown in FIG. 22, the compressed data inputted are decoded by the decoder 31, while being fed to the inverse quantization factor output portion 34, so that an inverse quantization factor to be used in the inverse quantization is extracted. The decoded data outputted from the decoder 31 are fed to the inverse quantizer 32, and the inverse quantizer 32 inversely quantizes the decoded data by using the inverse quantization factor fed from the inverse quantization factor output portion 34. The output from the inverse quantizer 32 is fed to the inverse DCT portion 33, and is subjected to inverse DCT by the inverse DCT portion 33, thereby becoming image data.
In the case where the above-described static image compression processing and the decompression processing as described above are performed, the compression ratio can be set frame by frame. However, in the case where the compression is performed with a certain region in the frame being focused, if the compression ratio is set relatively lower in accordance with the focused region, the image quality of the non-focused regions improves more than necessary. To the contrary, in the case where the overall compression ratio is set relatively higher, a sufficient image quality cannot be obtained with respect to the focused region.
Likewise, if the above-described method is applied without modification to the compression and decompression of image data in an ultrasonic diagnostic device using a convex-type probe, the inconvenience owing to the variation of the acoustic line density as described above occurs. For example, in the case where the compression and encoding of image data using a quantization table commonly is applicable to one entire frame, if the compression ratio is adjusted so as to be suitable for a region with a higher acoustic line density, the image quality of regions with lower acoustic line densities is impaired. To the contrary, if the compression ratio is adjusted so as to be suitable for a region with a lower acoustic line density, codes in an amount more than necessary are used in regions with higher acoustic densities.
As a method to cope with these problems, a method as follows is available: a compression ratio commonly applicable to an entire frame is not used, but one frame is divided into blocks and the compression ratio is switched block by block. This method is included in the JPEG extended standard. More specifically, the quantization table to be used is designated for each block, and selection information of the quantization table is encoded, so that the compression ratio is adjusted for each block.
Further, as shown in FIG. 21, a value obtained by multiplying a value obtained from a quantization table by a scale factor is fed as a quantization factor to the quantizer so that the step width in the quantization is adjusted. It is possible to vary this scale factor block by block and encode the value of the scale factor, so as to adjust the compression ratio for each block.
However, these methods require the encoding of compression parameters such as selection information of a quantization table or a scale factor, and hence the amount of codes may increase for the same, whereby the data amount as a whole also may increase.
Then, another method has been proposed, in which using the high correlation between the scale factor or the selection information of the quantization table for each block and a DC differential that is a differential between DC coefficients of blocks after DCT, the DC differential information and the scale factor information are combined and encoded, whereby necessary compression parameters are encoded efficiently (see e.g. JP 2000-92330 A).
FIG. 23 is a block diagram illustrating a configuration of an image data compression unit utilizing the foregoing method. In this method, as shown in FIG. 23, image data are processed by the block divider 11, the DCT portion 12, and the quantizer 13. The quantization step width is determined by multiplying the matrix of the basic quantization table 18 by a scale factor computed by a scale factor computation circuit 81. A quantified AC component is encoded by an AC component encoding circuit 83. A quantified DC component is converted into a DC differential by a DC differential computation circuit 84, and further, converted into a group number and an additional bit by a grouping circuit 85. The scale factor also is converted into a scale factor differential by a scale factor differential computation circuit 82, and further, converted into a group number and an additional bit by a grouping circuit 86. The group numbers of the DC component and the scale factor are encoded by a two-dimensional Huffman encoding circuit 87, and are outputted after each code element thereof is multiplexed by a multiplexing circuit 88.    Patent Document 1: JP 2000-92330 A